The present invention relates to the use of electromagnetic energy to assist in the recovery of organic and inorganic materials (for example, liquids and gases) from subsurface formations (for example, oil shale, tar sands, heavy oil, sulfur and other bituminous or petroliferous deposits) and, in particular, to an in situ electromagnetic apparatus, and a method of use thereof, for simultaneously heating and recovering organic and inorganic materials in a single borehole or a multiple borehole system.
The large scale commercial exploitation of certain subsurface mineral formations has been impeded by a number of obstacles, particularly the cost of the extraction and the environmental impact of above-ground mining. Organic material such as oil shale, tar sands, coal, and heavy oil can be subjected to heating to develop the porosity, permeability and/or mobility necessary for recovery. The high viscosity of bitumen and heavy oils in their native condition makes these substances extremely difficult to recover from subsurface formations. For example, it is not economically feasible to recover bitumen from tar sands by strip-mining and above-ground processing. Although in situ processing based on conventional (that is, non-electromagnetic) heating methods would have economic advantages and avoid severe environmental problems, all conventional in situ techniques are inadequate because of the difficulty in transferring heat through the subsurface mineral formation (since the mineral deposits are poor thermal conductors and are often impermeable to fluids). This problem is avoided by using electromagnetic methods of heating.
Previous efforts have been proposed to heat large volumes of subsurface formations in situ using electromagnetic energy. Investigators have explored the technical feasibility of using radio frequency energy for the volumetric heating of Utah tar sands. In order to achieve reasonable rates of product recovery by in situ tar sand processes, it is necessary to lower the viscosity of the bitumen (the rate of flow of bitumen within the deposit is inversely proportional to the viscosity). For example, the viscosity of bitumen from Utah tar sand deposits is greater than 10.sup.6 centipoise (cp) under reservoir conditions, and can be reduced to about 100 cp by heating the deposits at 125.degree.-150.degree. C. Under these conditions, the bitumen can be recovered either by gravity drive, gas injection, or by replacement of the bitumen with a suitable subsurface solution (liquids or gases). Alternatively, the bitumen can be pyrolyzed in situ and the oil product recovered by gas expansion and gravity drive. Prior electromagnetic methods also describe a transmission line system which is essentially a triplate structure composed of many closely spaced electrodes. Although this system demonstrates the ability of electromagnetic energy of appropriate frequency to heat tar sand material to elevated temperatures, product recovery is still required.
The stimulation of production from individual wells in heavy-oil deposits is generally difficult because the liquid flow into the borehole region may be impeded by the high viscosity of the oil, the precipitation of paraffin from the rock matrix, or the presence of water sensitive clays. The application of a modest amount of electromagnetic energy for heating around and away from the borehole will reduce the viscosity of the heavy oil. As a result, the liquid flow pattern will improve and the pressure gradient around the borehole will be reduced, thereby increasing overall production rates. Even greater increases in flow rates can be achieved by extending the heating patterns further out into the deposit by either lowering the radio frequency or by using more than one apparatus.
There has been considerable interest in developing in situ techniques in which electrical energy is employed to heat the borehole and through conduction to heat the subsurface formation to recover useful fuels. These approaches have not been successful because (i) they failed to heat the particular resource in significant volume and/or (ii) they depended upon ambient water to provide electrical conductivity. For example, one technique describes simple electrical heating elements which are embedded in pipes and the pipes inserted in boreholes in oil shale. Although this approach is technically feasible, it creates a very high temperature gradient around the boreholes. This results in an inefficient use of the applied energy, a very low level of useable heat per borehole and, consequently, a requirement for very closely spaced boreholes.
Alternative electrical in situ techniques have been proposed wherein the electric conductivity of the subsurface formation is relied upon to carry an electric current between electrodes inserted in separated boreholes. For example, sixty cycle (Hz) ohmic heating methods have been proposed in which electrical currents are passed through a tar sand deposit. As typically described, a simple pair of electrodes is placed into a subsurface mineral deposit and a 60 Hz voltage is applied. However, this technique is problematic: AC current will flow between the electrodes because the presence of water in the deposit allows mobile ions to lower the observed electrical resistance. Then, as heating continues, high current densities near the electrodes evaporate the local moisture, thereby terminating the heating process. Attempts to mitigate this effect have included injecting saline water from the electrodes and pressurizing the deposit to suppress vaporization. Even if these techniques were successful, the current density would be higher near the electrodes. This would cause inefficient transfer of electrical energy and result in unfavorable economics. Furthermore, many tar sand deposits are poor candidates for this technique because they have a low moisture content which prevents a reduction in electrical resistance, and a thin overburden which makes pressurization difficult.
Techniques for in situ oil shale retorting by employing radio frequency energy have been described in the patent literature. Some of these techniques use borehole applicator systems which have been successfully tested in the field for kerogen heating and subsequent oil recovery. The efficient transfer of RF energy away from the boreholes was accomplished through the appropriate choice of frequency, applicator design and input power control. During power application, initial heating occurred near the boreholes with attendant oil recovery followed by much large volumetric heating between boreholes. In some instances, the resulting oil product has been recovered by the antenna acting as an extractor. Oil vapor pressure and injected gas flow have been employed to assist in product recovery.
Thus, it is an object of the present invention to provide an electromagnetic apparatus, and a method of use thereof, for generating near-uniform heating of subsurface formations and simultaneously recovering organic and inorganic materials through the apparatus itself.
It is another object of this invention to provide a flexible or semi-rigid electromagnetic apparatus for simultaneously heating and recovering organic and inorganic materials in substantially horizontal boreholes.
It is yet another object of this invention to provide a phase-modulated multiple borehole system, and a method of use thereof, for generating near-uniform heating and simultaneously recovering organic and inorganic materials from larger subsurface formations and for creating steerable and variable heating patterns.
It is still another object of this invention to provide an electromagnetic apparatus, and a method of use thereof, for recovering oil trapped in rock formations.
It is still yet another object of this invention to provide an electromagnetic apparatus, and a method of use thereof, for decontaminating a region of the earth contaminated with hazardous materials.